This invention is related to optical devices and, more particularly, to narrow band optical filters.
In many optical technology applications, there exists a need to isolate a particular wavelength of light from incident light containing a broadband of wavelengths. In the field of infrared detection, for example, a target item which must be detected, such as a particular model or type of aircraft, frequently will emit a characteristic infrared radiation spectrum containing identifiable features, such as a peak amplitude at a particular wavelength. Thus, it is desirable in detection systems to filter incident light and thereby obtain a signal with a narrow bandwidth centered about a desired wavelength and with a high rejection for the off band portion of the signal.
One filtering technique practiced in the art to obtain such a result utilizes the optical properties of a birefringent crystal. A birefringent or doubly refracting crystal will divide a light beam travelling through it into fast and slow wave components which propagate at different speeds within the crystal. If the crystal is manufactured with the proper thickness and placed with the proper orientation, it can be made to act as a half-wave plate for light of a particular wavelength .lambda.. Furthermore, if such a half-wave crystal is sandwiched between a pair of crossed polarizers, the combination will act to filter broad band light traveling through it in such a manner that light at the particular wavelength .lambda. (and other wavelengths for which the phase retardation caused by the crystal is 180.degree.) will be preferentially transmitted. The separation of the bands thus produced in the transmitted spectrum by such an optical device will be inversely proportional to the thickness of the crystal.
In the design of a particular type of device, known as the Lyot-Ohman filter, a number of such crystal and polarizer combinations are aggregated with each crystal followed by one half as thick, i.e., the crystals are arranged so that the ratio of their thicknesses is 1:2:4:8, etc. With this configuration, every other maximum in the spectrum transmitted by the thickest crystal will be suppressed by a minimum in the next thinner crystal, and so on, so that it is possible with such an arrangement to isolate a few very narrow band wavelengths, which is a desirable and useful result. The Lyot-Ohman design, however, unfortunately tends to exhibit high reflection losses because of the large number of surfaces through which the incident light must pass, which results in a relatively low transmittance value for the filter. Furthermore, the large number of stages which are required in this type of filter to achieve a suitable narrow bandwidth result in a relatively bulky overall structure.
Therefore, a need has developed in the optical filtering art for a narrowband optical filter which will operate with a relatively high transmittance yet which is relatively compact and simple in its design.